Supercapacitors (sometimes referred to as ultra-capacitors) have received much attention recently in the technical literature, in industry and in the venture capital community, as a potential energy storage medium. This technology promises to provide a high power density (in units of kW/kg) and long cycle life (0.5-1.0 million cycles) while providing reasonable energy density (kWh/kg units). A number of companies, such as Maxwell (San Diego, USA), and Panasonic, Asahi Glass, Matsushita, NEC, and Nippon Chemicals and others, have been actively engaged in commercialization as well as advanced development. Although the technology has been around for thirty years, only recently commercial acceptance has arrived due to acceptable reliability and performance, at least for some applications such as memory backup and niche markets such as standby power mode in copying machines.
For many other commercial applications including hybrid/electric vehicles and military applications, development has a long way to go in terms of balance between power and energy densities, reliability, size, mass, safety and above all, price. Innovations in the electrode materials, electrolytes, other components, cell construction and almost everything else on cell design are on the table for further improvement. The focus in recent years has been on exploitation of nanostructured materials and principles of nanotechnology to improve the supercapacitor performance. The result so far has been promising and the performance has been steadily climbing. Nanostructured materials offer a high surface area and useable porosity for a given volume and mass, both of which are highly desirable for supercapacitor operation, which is the focus of this project.
Specifically, multiwalled carbon nanotubes (MWCNTs) have been grown in towers directly on metal alloys like nichrome, kanthal and stainless steel to reduce interface resistance; the nanotube towers will be treated quickly to improve surface wettability by the electrolyte; and if the design warrants, a psuedocapacitance component will be added by coating the MWCNT or SWCNT tower with an electrically conducting polymer (ECP). To understand all this, some background information is appropriate.
FIG. 1 (A. G. Pandolfo and A. E. Hollenkamp, Jour. Of Power Sources, vol. 157 (2006) p 11) graphically compares the specific power (Watts/Kg) versus specific energy available (Watt-hr/Kg) for four classes of energy storage devices: capacitors, electrochemical capacitors, batteries and fuel cells. High specific power and high specific energy available appear to vary inversely with each other so that one cannot have both in a single device.
Supercapacitors attempt to combine the best of capacitors and batteries to create an alternative form of energy storage device. Conventional capacitors provide a very high specific power exceeding 100 KWatts/Kg and long cycle life. The long life is due to the fact there are no chemical reactions and associated decays. However, the energy density of a commercial capacitor is small, only tens of mWatt-hr/Kg. At the other extreme, batteries provide high energy density, about 100 Watt-hr/Kg, but battery power density is about 100 Watts/Kg. An additional issue with batteries is the anticipated cycle life, limited by the chemical interconversions and concomitant phase changes. The supercapacitor, which is a hybrid between a battery and a capacitor, is not new as the first patent to SOHIO was granted in 1966 and NEC first marketed it for memory backup applications 20 years ago.
Two types of supercapacitors are available, based on how energy is stored within each device: electrochemical double layer capacitors (EDLCs) and redox capacitors. A redox capacitor, also known as a pseudocapacitor, relies on electron transfer reactions (Faraday redox) that occurs during the charge/discharge cycle of the cell and is thus not an electrostatic. Most common redox capacitors rely on oxides, such as ruthenium oxide and manganese oxide, as well as electrically conducting polymers (ECPs), such as polyaniline and polypyrrole. A chemical reaction-based operation in pseudocapacitors more nearly resembles a battery than a capacitor in its operation. In the EDLC, a pair of symmetric electrodes, usually carbon, separated by a porous medium is soaked in an electrolyte. When the electrodes are biased, ions move towards the opposite polar electrodes and charge separation is confined to a very thin region near the electrode called a double layer. In this sense, each electrode-electrolyte interface is a capacitor and, therefore, the device shown in FIG. 2 consists of two capacitors in series, with a circular pattern of high density MWCNTs. The cell capacitance is then given by1/C=1/C1+1/C2,  (1)where C1 and C2 are capacitance values of two adjacent electrodes, each given byC=∈A/d  (2)where ∈ is a dielectric constant for the material, A is the surface area of the carbon electrode and d is the double-layer thickness. When the electrodes are symmetric, the total capacitance is half that of a single electrode. If one electrode is far smaller than the other, the total capacitance is approximately the smaller of the two capacitance values. The energy, E and the power, P of the supercapacitor are given by:E=CV2/2,  (3)P=V2/4R,  (4)where C is capacitance, V is cell voltage, and R is the equivalent series resistance (ESR).
A capacitance value is primarily determined by the surface area and pore volume. Many carbon materials, such as activated carbon and carbon aerogel, have very large surface areas (≈2000 m2/gm). However, carbon materials often suffer from a significant fraction of unusable nanopores, which are pores with diameters 2 nm or less; mesopore diameters are 2-50 nm and macropore diameters are greater than 50 nm. The nanopores contribute heavily to the measured surface area but may not contribute to increasing the capacitance. Ion transport through such small pores may be restricted. Mesopores are the most ideal for supercapacitor operation. Therefore, a simple metric of large surface area from adsorption isotherm measurements alone is not adequate to evaluate various carbon forms for capacitance enhancement; pore size distribution must also be considered.
A capacitor operating voltage is determined, in part, by the choice of the electrolyte, because electrolyte stability is severely compromised above certain voltages. Aqueous electrolytes, such as acids, have an operating voltage of only 1.0-1.5 Volts but are inexpensive and exhibit high ionic conductivity. Numerous nonaqueous electrolytes, such as polycarbonate and acetonitrile, allow higher operating voltages, for example 2.5 Volts. However, their electrical resistivity is at least one order of magnitude higher than the aqueous electrolytes. According to Eq. (4), a high value for R is detrimental for obtaining high power. R consists of several contributions:R=Rc+Rem+Rint+Relec+Rion+Rsep  (5)where Rc is collector metal resistance, electrode material (carbon) resistance, Rem; is resistance of the interface between the carbon and the current collector metal, Rint is electrolyte resistance, Rele, is resistance due to ion transport through the pores, Rion, and Rsep. is separator resistance.
What is needed is a capacitor device that allows adequate transport between capacitor electrodes but suppresses electrical shorting between electrodes, that has a relatively low interface resistance between each electrode and any substance that physically separates the electrodes, and that has reduced capacitance, where the separator includes apparatus that can be made hydrophilic.